1. Field of the Invention
The present invention relates to an image correcting apparatus and method for subjecting digital image data to image correction, and a program for causing a computer to make image correction. Further, the present invention relates to a look-up table creating apparatus and method related to the brightness of an image, and a program for creating a look-up table.
2. Description of the Background Art
Digital image data is subjected to various types of correction processing for the purpose of making an image represented by digital image data easy to see and improving the quality of the image, for example. Concentration correction (luminance correction, brightness correction) which is the one correction processing is for correcting the digital image data such that the image represented by the digital image data becomes brighter when it is dark, while becoming darker when it is bright.
JP-A-2000-196890 describes an image processing apparatus that converts, when the concentration of a subject is not within a target concentration range, the gray scale of an image such that the concentration of the subject will be within the target concentration range in a state where the concentrations of the highest concentration portion and the lowest concentration portion in the image are substantially held. A gray-scale curve for concentration correction is specifically shown in FIGS. 3 and 4.
In the image processing apparatus (concentration correction conforming to the gray-scale curve shown in FIGS. 3 and 4) described in JP-A-2000-196890, the concentration of the subject can be within the target concentration range, so that the concentrations of the highest concentration portion and the lowest concentration portion in the image are not corrected. Since the possibilities that the concentration of a highlight portion in the image is reduced and the concentration of a shadow portion in the image is increased are eliminated, the image can be prevented from having a loose impression.
However, the gray-scale curve not only defines the concentration (brightness, darkness) of the image after correction but also is associated with high contrast (hard gradation) or low contrast (soft gradation) of the image. If the slope of the gray-scale curve is large, a dark portion and a bright portion in the image respectively become darker and brighter, so that the image after correction becomes a sharp image (high contrast, hard gradation). Conversely, when the slope of the gray-scale curve is small, the difference between a dark portion and a bright portion in the image becomes small, so that the image after correction has a blurred impression (low contrast, soft gradation).
The image processing apparatus (concentration correction conforming to the gray-scale curves shown in FIGS. 3 and 4) described in JP-A-2000-196890 does not consider the change in the high contrast or low contrast of the image. When the concentration correction conforms to the gray-scale curve shown in FIG. 3, the image after correction always has low contrast with respect to a dark portion (the input concentration is 0 to 1) in the image, while always having high contrast with respect to a bright portion (the input concentration is 1 to 2) in the image. When the concentration correction conforms to the gray-scale curve shown in FIG. 4, the image after correction always has high contrast with respect to a dark portion (the input concentration is 0 to 1) in the image, while always having low contrast with respect to a bright portion (the input concentration is 1 to 2) in the image.
Furthermore, in a case where color prints are produced from images, for example, represented by digital image data obtained by reading images recorded on photographic negative films or reversal films using a scanner or the like or digital image data acquired by a digital camera, the digital image data may, in some cases, be subjected to brightness correction (luminance correction, concentration correction) such that the brightness (luminance, concentration) of a desired subject image portion such as the face of a character included in a print image becomes suitable brightness (luminance, concentration). In JP-A-2000-196890, the face of a character included in the image is extracted, and the image data is subjected to concentration correction such that the average concentration in a face area will be in a correction target range.
When the image is corrected to higher brightness, however, noise included in the image is liable to be viewed more conspicuously than that before correction. That is, consider a case where the entire image is uniformly subjected to concentration correction (such correction that the image becomes brighter) such that the average concentration in a desired subject image portion such as the face of the character included in the image (the average concentration in the face area) is in the correction target range, as in JP-A-2000-196890. In this case, when the desired subject image portion in the image includes much noise, it becomes brighter by the concentration correction. However, the noise is conspicuous in the image. If a portion other than the desired subject image portion includes noise, the noise is also conspicuous with respect to the portion.
When image data to be processed includes noise, a noise component in the image data can be removed by using a noise removal filter. However, it is difficult to completely remove the noise. When image data having a high resolution (the number of pixels is large when it is displayed) is subjected to noise removal processing, a long processing time period is required, so that the resources of a computer device is occupied for a long time.